1. Field of the Invention
The present invention relates to electrodeposition, and more particularly, relates to cationic electrodeposition of aqueous dispersions of cationic resinous compositions.
2. Brief Description of the Prior Art
Cationic electrodeposition has been used industrially since 1972. The early cationic electrodeposition compositions comprised quaternary ammonium salt group-containing resins in combination with aminoplast curing agents. In 1976, cationic compositions comprising amine salt group-containing resins in combination with blocked isocyanate curing agents were introduced for priming automobile bodies. Today, over 90 percent of the automobile bodies are primed by cationic electrodeposition and practically all of the cationic compositions use the amine salt-blocked isocyanate resins.
In cationic electrodeposition, the part being coated is of course the cathode. The counter-electrode or anode is usually made of a corrosion-resistant material such as stainless steel since most cationic electrodeposition baths are acidic in nature. Because of the electrochemical reactions which occur at the anode, the stainless steel electrode slowly dissolves during the cationic electrodeposition process. The rate of dissolution depends principally on the current density, temperature of the electrodeposition bath to which the anode is exposed; the greater the current density and the higher the temperature, the faster the rate of ion dissolution. Also, the composition to which the electrode is exposed can affect the rate of dissolution. The presence of chloride ion greatly accelerates dissolution, and other unknown constituents of the electrodeposition bath can also affect dissolution. It has been found, for example, that electrodeposition baths in one location may be relatively passive to the stainless steel anodes, whereas electrodeposition baths in another location employing the same cationic paint may be very aggressive towards the stainless steel anode. The dissolution of the anode results in low film builds and poor appearance. Eventually, if the dissolution is great enough, the anode must be replaced resulting in a time-consuming and expensive shut down of the electrodeposition process.
It is an object of the present invention to overcome the above problems and to provide for a method of cationic electrodeposition with an anode which is resistant to deterioration and dissolution in all cationic electrodeposition environments. Practicing cationic electrodeposition in this manner would insure consistent results in terms of coating quality and would also result in considerable savings from not having to replace the anodes because of dissolution.